1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus, which includes a heater having a heat generating resistor provided on a substrate thereof and a fixing film configured to rotate while being in contact with the heater, and configured to thermally fix a tone image formed on a recording paper by the heat from the heater applied via the fixing film onto a recording paper.
2. Description of the Related Art
A film type fixing unit has been widely used as a fixing unit of an electrophotographic copying machine or a printer, which includes a heater having a heat generating resistor provided on a ceramic substrate, a film (flexible member) configured to move while being in contact with the heater, and a pressure roller configured to form a nip portion with the heater via the film.
A film type fixing unit thermally fixes a toner image formed on a recording paper onto the recording paper by pinching and conveying a recording paper bearing an unfixed toner image into a nip portion of the fixing unit and applying heat to the toner image on the recording paper. In the above-described fixing unit, the time taken from the start of supply of power to the heater to the timing at which the temperature of the heater reaches a fixable temperature is short. Accordingly, in a printer including the above-described fixing unit, the time taken from the timing of input of a printing command to the timing of output of the first image (a first print out time (FPOT)) is short. In addition, the power consumption of the printer consumed during waiting for a printing command is small.
In an image forming apparatus including the above-described film type fixing unit, when printing on a small size paper such as a postcard, whose width is smaller than a maximum printable recording paper width, the film type fixing unit may damage the film or the pressure roller if the temperature of the fixing unit is excessively raised because of the temperature rise in an area of the heater through which no recording paper passes (a non-paper-passage area) (i.e., if a phenomenon so-called “temperature rise of non-paper passage area” occurs).
Japanese Patent Application Laid-Open No. 04-237084 discusses that if the magnitude of the heat generating quantity per unit length is reverse to the magnitude of the nip width in a longitudinal direction of the heater, then the temperature rise in the non-paper-passage area, which may occur when a small size paper is fed, can be effectively reduced.
Meanwhile, standard size papers of various sizes are used in different countries and regions. In North America, a letter (LTR) size paper (approximately 8.5×11 inches (≈216×279 mm)) is widely used. Accordingly, in North America, an image forming apparatus in which the width of the LTR size paper is set as the maximum printable recording paper width is used.
On the other hand, In Europe and Asia, an A4 size paper (210×297 mm), whose width is smaller than that of the LTR size paper, is widely used. Accordingly, in Europe and Asia, an image forming apparatus in which the width of the A4 size paper is set as the maximum printable recording paper width is used.
Under these circumstances, in order to achieve commonality of the type of an image forming apparatus to be distributed within various countries and regions and reduce the costs thereof, the image forming apparatus in which the size of the LTR size paper is set as the maximum printable recording paper width, which is used in North America, is also used in European and Asian countries and regions.
When printing is performed on an A4 size paper with an image forming apparatus adaptable to the LTR size paper, the temperature rise in non-paper passage areas may occur. The temperature may not exceed the heat resistant temperatures of the parts and components of the fixing unit if printing is executed at a conventional printing speed even when the temperature rise in non-paper passage areas occurs. However, in recent years, the heat capacity of a fixing unit has been decreased and the fixing unit has been configured to have a heat-insulated structure in order to save energy consumption, while the fixation temperature has been raised to increase the printing speed.
Accordingly, if the printing speed is set to a speed higher than the printing speed of a currently marketed image forming apparatus, if an A4 size paper is used for an image forming apparatus in which the size of the LTR size paper is set as the maximum printable recording paper width, the temperature of the non-paper-passage area may rise to a temperature higher than the heat resistance temperature of the part and components of the fixing unit.
The temperature rise in non-paper passage areas may become particularly serious in executing one edge-aligned paper passage, in which an A4 size paper is fed aligned with one edge of the paper passage area in a state in which a recording paper position regulation member of a paper cassette is set to the size of the LTR size paper. In this case, the film or the pressure roller may be damaged.
The one edge-aligned paper passage will be described in detail below. When setting a paper on a paper feed cassette, a user of the image forming apparatus sets two recording paper position regulation members, which are provided opposite each other, according to the size of the paper to be used to regulate both side edges of the paper in the lateral (widthwide) direction. By regulating the position of the paper in the widthwide direction, an image can be formed on an appropriate position on the paper.
FIG. 13 illustrates a positional relationship between the heater and the paper in the longitudinal direction and the widthwide direction.
In printing on an A4 size paper, the user sets the recording paper position regulation member to the position of the A4 size paper (to a position P2 in FIG. 13). Accordingly, the position of the paper in the widthwide direction is regulated. Thus, the edge of the A4 size paper is fed aligned to a position approximately 3 mm closer to the center of the surface of the heater compared to the position of passage of the edge of an LTR size paper P1. In this case also, the temperature of the non-paper-passage area of the heater may slightly rise, but the temperature may not be excessively raised to a high temperature at which the film or the pressure roller may be damaged.
However, in printing on an A4 size paper, if the user sets the recording paper position regulation member aligned to the size of an LTR size paper (at a position P3 or P4 in FIG. 13) by mistake and if the paper is fed in a state in which the side edge of the paper contacts either one of the two recording paper position regulation members, then the non-paper-passage area becomes large. Accordingly, in this case, the temperature of the heater may easily become very high.
In order to prevent the above-described phenomenon, it may be useful in reducing the temperature rise in non-paper passage areas to reduce the length of the heat generating resistor in the non-paper-passage area. However, if the length of the heat generating resistor in the non-paper-passage area is reduced to be very short, the fixing property on the edge of an LTR size paper may degrade.
In addition, by reducing the speed of conveyance of a recording paper (i.e., by taking a large distance between printing papers), the temperature rise in non-paper passage areas may be reduced. However, in this case, because the printing speed decreases, the convenience of users in European and Asian countries or regions, who often use an A4 size paper, may degrade.
The problem may occur not only in an image forming apparatus in which the size of the LTR size paper is set as the maximum printable standard size paper width but also in an image forming apparatus in which the size of a paper other than an LTR size paper is set as the maximum printable standard size paper width.
More specifically, the above-described problem may occur if printing on a ledger size paper (11×17 inches (≈279×432 mm)), which is often used in North American countries or regions, is executed with an image forming apparatus in which the size of an A3 size paper (297×420 mm), which is often used in European or Asian countries or regions, is set as the maximum printable standard size paper width.